Abstract
Growth cones from freeze-substituted intact chick optic tectum were analyzed in serial reconstructions of thin-section electron micrographs in order to determine which internal organelles might contribute membrane for plasmalemmal expansion. These growth cones contain numerous stacked and single lumenless membrane-limited disks; the stacks are arrays of single disks interconnected, and possibly organized, by intervening electron-dense cross-links. The single and stacked disks together account for 80% of the total intracellular membrane in the growth cones. Single disks frequently lie close to and occasionally contact the filopodial plasmalemma; regularly spaced electron-dense cross-links also occur at these juxtapositions between single disks and the plasmalemma. Some of the juxtaposed disk membranes contact the growth cone plasmalemma, and images of some of these contacts appear to indicate fusion of the disk membrane with the plasmalemma. When excised optic tecta are exposed to cationized ferritin for various times, ferritin micelles appear sequentially in coated pits, coated vesicles, smooth vesicles, vacuoles, and then in stacked and single disks, including some of those contacting the plasmalemma. Because the cytoplasmic disks filled only at the longest times after exposure to ferritin, the membrane continuities between the disks and the plasmalemma are thought to be indicative of exocytosis rather than endocytosis. We propose, therefore, that components of the plasma membrane are recycled through the stacks of lumenless disks in the chick tectal growth cones; the disks therefore represent a pool of internal membrane waiting to be added to the growth cone plasmalemma that could be used for filopodial extension or neuritic extension.
